Automatic gauge control by tension for tandem rolling mills

ABSTRACT

Method and apparatus for varying the gain of an automatic gauge control loop for rolling mills wherein gauge is controlled by varying tension in the rolled strip between the last two stands of a tandem rolling mill. The gain of the loop is varied as a function of transport time between the bite of the rolls in the last stand and a thickness gauge positioned beyond the last stand. At low mill speeds (i.e., long transport times), the gain of the loop is maintained low and varied as a function of the cross-sectional area of the strip between the last two stands. At high mill speeds (i.e., short transport times), the gain of the loop is increased and varied as a function of both crosssectional area and the speed of the last stand.

United States Patent .[1'91 7 Peterson Oct. 16, 1973 1 AUTOMATIC GAUGECONTROL BY TENSION FOR TANDEM ROLLING MILLS [75] inventor: Robert S.Peterson, Williamsville,

[22] Filed: Feb. 29, 1972 [21] Appl. N0.: 230,299

[52] US. Cl. 72/9, 72/12 [51] int. Cl. B21b 37/02 [58] Field of Search72/8, 9, 10, 11, 72/16, 12

[56] References Cited UNITED STATES PATENTS 3,049,036 8/1962 Wallace etal. 72/9 7/1962 Wallace et a1. 72/9 3,492,844 2/1970 Silva ..72/83,158,049 11/1964 Huntley ..72/8

Primary Examiner--Milton S. Mehr Attorney-F. H. Henson et al.

[5 7] ABSTRACT Method and apparatus for varying the gain of an automaticgauge control loop for rolling mills wherein gauge is controlled byvarying tension in the rolled strip between the last two stands of atandem rolling mill. Thegain of the loop is varied as a function oftransport time between the bite of the rolls in the last stand and athickness gauge positioned beyond the last stand. At low mill speeds(i.e., long transport times), the gain of the loop is maintained low andvaried as a function of the cross-sectional area of the strip betweenthe last two stands. At high mill speeds (i.e., short transport times),the gain of the loop is increased and varied as a function of bothcrosssectional area and the speed of the last stand.

8 Claims, 4 Drawing Figures FIG] SPEED REGULATOR TENSION REGULATOR 22AUTO MATIC v GAUGE CONTROL TENSION REFERENCE GAUGE REFERENCE PATENTEDUBT1 sum 'SHEEI 10F 2 AUTOMATIC GAUGE CONTROL BY TENSION FOR TANDEM ROLLINGMILLS CROSS REFERENCES TO RELATED APPLICATIONS Application Ser. No.230,298 and application Ser. No. 230,300 both filed concurrentlyherewith and assigned to the assignee of the present application.

BACKGROUND OF THE INVENTION As is known, the gauge of the strip materialpassing through a tandem rolling mill can be varied by varying thetension in the strip between the last two stands in the mill. Actualtension between the last two stands is compared with desired tension fora specified gauge;

and if the two are not the same, the speed of the last stand is varied.This changes the mill stretch of the last stand which changes the rollgap between the last stand work rolls until the strip is at the desiredgauge.

In a tension gauge control loop of this type, the strip material, afterpassing through the bite of the rolls in the last stand, must travel toa thickness gauge positioned, for example, about five feet beyond thebite of the rolls. Thus, a deviation in gauge from desired gauge is notdetected until the strip material has traveled five feet to thethickness gauge which then develops an error signal used to takecorrective action. This gives rise to what is known as transport timerequired for the strip to travel between the bite of the rolls and thethickness gauge. At low speeds and long transport times, the responsetime or gain of the control loop should be low. Otherwise, because ofthe long time delay between detection of a gauge error and correction, ahigh gain system would become unstable (i.e., oscillate). At highspeeds, on the other hand, the response time or gain of the control loopcan and should be increased to achieve better gauge control.

In the past, automatic gauge control loops of this type did not providefor a change in gain with changes in the product being rolled or theoperating speed of the mill. The automatic gauge control loop wasadjusted for the worst possible condition which is for large stripcrosssectional area and low mill speed operation, on the order of about500 feet per minute. Below this mill speed of 500 feet per minute, theautomatic gauge control loop could not be utilized; and at high millspeeds and/or small strip cross-sectional areas, the response of theautomatic gauge control loop was very slow.

SUMMARY OF THE INVENTION In accordance with the present invention, amethod and apparatus are provided for controlling gauge at the output ofa tandem rolling mill by varying the tension between the lasttwo standsin the mill, and wherein the gain of an automatic gauge control loop forvarying the last stand speed is varied as a function of stripcrosssectional area at low speeds and as a functionof stripcross-sectional area and of the speed of the mill at high speeds. Inthis manner, the gain of the loop gradually varies upwardly as the speedof the mill increases, thereby insuring optimized gain characteristicsunder all operating conditions.

Specifically, the invention involves l) measuring the gauge of stripmaterial issuing from the last stand of a tandem rolling mill at a pointremoved from the last stand and producing a signal proportional to theactual measured gauge, (2) comparing the actual gauge signal with adesired gauge signal as determined by an operator to derive a gaugedeviation signal for varying the tension between the last two stands byvarying the speed of the last stand, (3) multiplying the gauge deviationsignal by the cross-sectional area of the strip material being rolled toderive a first error signal, (4) multiplying the first error signal bythe speed of the last stand to derive a second error signal, and (5)summing said first and second error signals and utilizing the sum tocontrol the speed of the last stand and, hence, the tension between thelast two stands.

At low strip speeds, the first error signal is effective to control thelast stand speed and, hence, tension between the last two stands. On theother hand, at low speeds, the second error signal is negligible sinceit is the product of the last stand speed (which is small and attenuatesthe error signal) and the gauge deviation signal. At high mill speeds,the second error signal which is the product of speed and gaugedeviation is greater than the first error signal and controls the gainof the loop at these high speeds.

The above and other objects and features of the invention will becomeapparent from the following detailed description taken in connectionwith the accompanying drawings which form a part of this specification,and in which:

FIG. 1 is a schematic diagram of a tandem rolling mill operationincorporating the automatic gauge control system of the presentinvention in block form;

FIG. 2 is a Bode plot of rolling speed versus gain, showing the mannerin which the gain increases as a function of speed in accordance withthe system of the invention;

FIG. 3 is a detailed block diagram of the automatic gauge control systemof the invention; and

FIG. 4 illustrates in greater detail the gauge control system of theinvention.

With reference now to the drawings, and particularly to FIG. 1, afive-stand tandem rolling mill is shown including five stands S1, S2,S3, S4 and S5. The strip inaterial 10 to be rolled passes between therolls of the successive stands 31-55 and is progressively reduced ingauge while the speed of the strip material increases at the output ofeach stand. The rolls for each of the stands are provided with drivemotors, only the motors M4 and M5 for stands S4 and S5 being shown inFIG. 1. Motors M4 and M5 are controlled by speed regulators SR4 and SR5,respectively, which receive a master speed reference signal on lead 12from a master mill speed controller, not shown. Suitable screwdownmechanisms and controls therefor are provided for each of the stands51-85. In accordance with the present invention, the spacings betweenthe work rolls of the last stand S5 are not varied by the screwdowncontrol during a rolling operation, the final output gauge beingcontrolled by varying stand S5 speed to vary the tension between thelast two stands as explained above.

The gauge of the strip material 10 issuing from the last stand S5 ismeasured by an X-ray gauge 14 or the like which produces a signal onlead 16 proportional to actual gauge. The signal from X-ray gauge 14 iscompared at summing point 18 with a gauge reference signal on lead 20determined by the operator of the mill, or possibly by a computer; thisgauge reference signal being proportional to the desired output gauge.If the desired gauge signal on lead 20 is not equal to the actual gaugesignal on lead 16, an error signal is developed which is then convertedto a percent error signal (volts per percent) based on the desireddelivery gauge, and is then applied to an automatic gauge controlcircuit 22, hereinafter described in detail.

Also applied to the automatic gauge control circuit 22 is a signalderived from a tachometer or pulse generator 23. This signal isproportional to the circumferential speed of the rolls in the last standS5 and, hence, the speed of the strip material issuing from the mill.The output signal from the automatic gauge control circuit 22 is thensummed at summing point 24 with a tension reference signal on lead 26and with an actual tension signal on lead 28 derived from a tensiometer30 in engagement with the strip material between the last stands S4 andS5. The signal from the gauge control circuit 22 and the tensionreference signal 26 are summed and compared in subtractive relationshipat point 24 with the actual tension signal from tensiometer 30. Theresulting signal is then applied as an error signal to a tensionregulator 32, the details of which may be had by reference to copendingapplication Ser. No. 230,300, filed concurrently herewith. Note that thetension reference signal is also applied to the automatic gauge controlcircuit 22 for a purpose which will hereinafter be described in greaterdetail.

As was explained above, it is necessary in accordance with the presentinvention to control the gain of the automatic gauge control loop at lowspeeds as a function of the cross-sectional area of the strip materialbeing rolled. Accordingly, means must be provided for determining thecross-sectional area of the strip material 10 between the last twostands. To this end, the gauge of the strip material between stands S1and S2 is measured by X-ray gauge 34 and applied to circuit 36 alongwith signals from tachometer generators or pulse generators 38 and 40.Tachometer generator 38 is connected to the rolls of the stand S1 and,hence, produces an output signal proportional to the speed of stand S1;whereas tachometer generator 40 is connected to the rolls of stand S4and produces an output signal proportional to the speed of stand S4.X-ray gauge 34, of course, produces a signal proportional to thethickness of the strip material between the first and second stands.

It is known that the width of strip material being rolled does not varymaterially in passing from one stand to the next. Furthermore, it isknown in accordance with the constant volume principle that the volumeof material entering the bite of the rolls ofa rolling mill is equal tothe volume of material leaving. That is,

where:

G, and G the gauges of the strip material entering and leaving thesecond stand S2, for example; V, and V the velocities of the stripmaterial entering and leaving the second stand S2; and W the width ofthe strip material. Consequently, by knowing the gauge of the stripmaterial between the first and second stands, the speed of the firststand, and the speed of the fourth stand, the area, A4, of the stripmaterial between the fourth and fifth stands S4 and 85 can be determinedfrom the equation:

Circuit 36, therefore, performs this computation and derives a signal onlead 42 proportional to A4, the area of the strip meterial betweenstands S4 and S5. This signal on lead 42 is applied to the automaticgauge control circuit 22 as well as the tension regulator 32.

The details of the automatic gauge control circuit are shown in FIGS. 3and 4. The gauge deviation signal from summing point 18 is applied toerror compensation amplifier 44 which produces a linear output signalvariable above and below the zero axis, depending upon the polarity ofthe deviation signal, and is limited at points above and below the zeroaxis as shown by the transfer characteristics on the block 44 of FIG. 3.The output signal from block 44, comprising a signal on lead 46proportional to deviation from desired gauge, is then multiplied inmultiplier 48 with the signal on lead 42 proportional to the area A,between the last stands S4 and S5. The output of the multiplier 48 isthen applied to a second multiplier 50 where it is multiplied with asignal V,, on lead 52 from the tachometer generator 23.

The signal at the output of multiplier 48 is applied through apotentiometer K2 to a correction amplifier 54; While that at the outputof multiplier 50 is applied through potentiometer Kl to the correctionamplifier 54. Note that the amplifier 54 has a tension reference signalapplied thereto via lead 56 for a purpose hereinafter described.

The output of the amplifier 54, comprising the original gauge deviationsignal as modified by multipliers 48 and 50, is applied to the summingpoint 24. Assuming that the tension signal from tensiometer 30 matchesthe tension reference signal on lead 26 or that the gauge of the issuingstrip material has deviated from desired gauge, the summation of thesignals at point 24 will produce an error signal which is applied totension regulator 32 to either increase or decrease the speed of thelast stand S5 via speed controller SR5. It should be understood,however, that the error signal can be applied directly to the speedcontroller without an intervening tension regulator, if the advantagesof the tension regulator, described in the aforesaid copendingapplication Ser. No. 230,300 can be sacrificed. In effect, an errorsignal from the automatic gauge control circuit 22 varies the tensionreference signal as applied to summing point 24. This, in turn, changesthe strip tension which causes a change in the roll force of the laststand which brings about a change in the mill stretch of the last stand.By changing the mill stretch in the last stand S5, the roll gap ischanged to bring the strip on gauge. At the same time, the change indelivery stand speed to provide the required tension between the lasttwo stands to bring the strip on gauge will give the correct speedrelationship between the delivery stand S5 and the other stands tomaintain the strip on gauge.

The details of the amplifiers 44 and 54 and the potentiometers K1 and K2are shown in FIG. 4. Amplifier 44 comprises an operational amplifier 58having two feedback paths including, respectively, a resistor 60 andlimiter 62 which limits the maximum output of the amplifier above andbelow the zero reference. One input to the operational amplifier 58 isconnected through resistors 64 and 66 to the summing point 18; while theother input to the amplifier 58 is connected through resistor 68 toground. The opposite ends of the resistor 64 are connected throughcapacitor 70 and resistor 72 to ground as shown.

Each of the potentiometers K1 and K2 is provided with a movable tapconnected through resistors 74 and 76, respectively, to a summing point78. Point 78 is connected through resistors 80 and 82 to ground. Point78 is also connected to one input of operational amplifier 84; while theother input to the operational amplifier 84 is connected throughresistor 86 to ground. The amplifier 84 is provided with a feedback pathincluding resistor 88 and capacitor 90. Also connectedv between theinput and output of amplifier 84 is a variable limiter 94 which, inresponse to a tension reference signal on lead 96, limits the maximumoutput of the circuit 54 as applied to the tension regulator 32 so asnot to exceed a maximum tension value and possibly cause a break in thestrip. In shunt with multiplier 48 is a potentiometer K3, the purpose ofwhich will hereinafter be described.

In the operation of the invention, the automatic gauge control loopresponse at low threading speeds is determined by the potentiometer gainsetting K2, which is a signal proportional to the gauge deviation signalmultiplied by the cross-sectional area of the strip between stands S4and S5. As will be appreciated, the signals from potentiometers K1 andK2 are summed at point 78 before being applied to circuit 54. However,at low speeds, the signal from potentiometer K1 is negligible since thesignal at the output of multiplier 50 is derived by multiplication bythe speed V, of stand S5 (which is small at low speeds and attenuatesthis control signal).

Since total tension in pounds is being controlled and not pounds persquare inch, which is the variable which directly affects strip gaugeand not total tension, the fixed plant of the automatic gauge controlloop has a gain which varies inversely with the cross-sectional areabetween the last two stands. As'will be appreciated, this gain variationin the fixed plant is compensated for by multiplexing the gain intheautomatic gauge control circuit 22 by the cross-sectional area betweenstands S4 and S5. Since the variation in strip cross-sectional area canbe as high as it is important that this gain compensation be included ina tension-type automatic gauge control loop. This latter feature adaptsthe loop to strip product variation.

At higher speeds (i.e., 500 feet per minute and above), the signal atthe output of the multiplier 50 becomes significant and in conjunctionwith the output of multiplier 48 controls the amplifier 54. Hence, atlow speeds, the gain of the gauge control loop is controlled by theproduct of the gauge deviation signal and strip cross-sectional area;whereas at very high speeds, above 2000 feet per minute, the gain of theloop is controlled primarily by the product of gauge deviation timescross sectional area, times speed of the strip.

At very light gauges, it is desirable to increase the gain of the loopover that which would be derived from the output of multiplier 48.Hence, a third potentiometer K3 is connected in shunt with themultiplier 48 and is adjusted, when rolling light gauges, to bypass thesignal around multiplier 48.

The remaining strip variable of the fixed plant which can vary is thestrip hardness gain function K Since very little reduction is taken onthe last stand of a tandem rolling mill (approximately 5 percent), andsince the strip hardness has probably reached its maximum value beforethe strip enters stand S5, this gain factor K, can be neglected in manycases. However, if it is desired to introduce the gain factor K,, it canbe introcontributed to the control loop by the speed regulator on thelast stand. Hence, at high speeds, a plot of loop gain versus speed isthat shown by curve 100 in FIG. 2. At low speeds, the transport timedelay between the delivery gauge 14 and stand S5, which is approximately7 2 seconds, limits the crossover frequency of the automatic gaugecontrol loop to approximately 0.5 radian per second, resulting in curve102 as shown in FIG. 2.

The present invention thus provides a means for controlling the gain ofan automatic gauge control loop for tandem rolling mills utilizing gaugecontrol by tension wherein the gain of the loop is varied as a functionof the cross-sectional area of the strip entering the last stand at lowspeeds, and by the speed of the last stand in addition to thecross-sectional area of the strip entering the last stand at highspeeds. Although the invention has been shown in connection with acertain specific embodiment, it will be readily apparent to thoseskilled in the art that .various changes in form and arrangement ofparts may be made to suit requirements without departing from the spiritand scope of the invention.

I claim as my invention:

1. In the method for controlling final output gauge of strip materialpassing through a tandem rolling mill by varying the tension in thestrip material between the last two stands in said tandem rolling mill,the steps of:

measuring the gauge of strip material issuing from said last stand at apoint removed from the last stand and producing a signal proportional tothe actual measured gauge,

comparing said actual gauge signal with a desired gauge signal to derivea percent gauge deviation signal for varying the tension between thelast two stands by varying the speed of said last stand, electricallycalculating the cross-sectional area of the strip material between saidlast two stands and modifying said gauge percent deviation signal as afunction of the cross-sectional area of the strip material thuscalculated to derive a first error signal,

modifying said first error signal as a function of the speed of saidlast stand to derive a second error signal, and

combining said first and second error signals and controlling the speedof said last stand as a function of the combined error signals.

2. The method of claim 1 wherein said combined error signals are addedto a tension reference signal and compared with a signal proportional toactual tension in the strip between the last two stands to derive asignal for controlling the speed of said last stand.

3. The method of claim 1 wherein said gauge deviation signal ismultiplied by the cross-sectional area of the strip material betweensaid last two stands to derive said first error signal and said firsterror signal is multiplied by the speed of said last stand to derive thesecond error signal.

4. The method of claim 1 wherein said gauge percent deviation signal issummed with said first error signal to produce a third error signal, andincluding the steps multiplying said third error signal by the speed ofsaid last stand to derive said second error signal, and running saidsecond and third error signals and using the sum to control the speed ofsaid last stand.

5. The method of claim 1 including the step of producing a signalproportional to the cross-sectional area of the strip material betweensaid last two stands by multiplying the gauge of the strip materialbetween the first two stands in said tandem rolling mill by the speed ofstrip material as the output of the first stand in the rolling mill, anddividing the product by the speed of the fourth stand in the tandemrolling mill.

6. Apparatus for controlling the final output gauge of strip materialpassing through a tandem rolling mill by varying the tension of thestrip material between the last two stands in said mill, comprising anautomatic gauge control loop including means for measuring the gauge ofstrip material issuing from said last stand at a point removed from thelast stand and for producing a signal proportional to the actualmeasured gauge, means for comparing said actual gauge signal with adesired gauge signal to derive a gauge percent deviation signal based ondesired delivery gauge, means for varying the speed of said last standto thereby vary the tension between the last two stands of the tandemrolling mill, means for generating a cross-sectional area signal whichvaries as the cross-sectional area of the strip material between thelast two stands of the mill varies, means for generating a signal whichvaries as a function of the speed of the last stand in the mill, meansfor modifying said gauge percent deviation signal as a function ofvariations in said cross-sectional area signal and said speed signal,and means for applying said modified gauge percent deviation signal tosaid means for varying speed whereby the speed of the last stand will bemodified in response to changes in both strip crosssectional area andstrip speed.

7. The apparatus of claim 6 wherein said means for modifying comprisesmeans for multiplying said gauge percent deviation signal by thecross-sectional area of the strip material between said last two standsto derive a first error signal, means for multiplying said first errorsignal by the speed of said last stand to derive a second error signal,and means for combining said first and second error signals and forapplying the combined error signals to said means for varying speed.

8. The apparatus of claim 6 including a speed regulator for said laststand, tension regulating circuit means having its output connected tosaid speed regulator, and means for applying the modified gauge percentdeviation signal to said tension regulating circuit means. a

1. In the method for controlling final output gauge of strip materialpassing through a tandem rolling mill by varying the tension in thestrip material between the last two stands in said tandem rolling mill,the steps of: measuring the gauge of strip material issuing from saidlast stand at a point removed from the last stand and producing a signalproportional to the actual measured gauge, comparing said actual gaugesignal with a desired gauge signal to derive a percent gauge deviationsignal for varying the tension between the last two stands by varyingthe speed of said last stand, electrically calculating thecross-sectional area of the strip material between said last two standsand modifying said gauge percent deviation signal as a function of thecross-sectional area of the strip material thus calculated to derive afirst error signal, modifying said first error signal as a function ofthe speed of said last stand to derive a second error signal, andcombining said first and second error signals and controlling the speedof said last stand as a function of the combined error signals.
 2. Themethod of claim 1 wherein said combined error signals are added to atension reference signal and compared with a signal proportional toactual tension in the strip between the last two stands to derive asignal for controlling the speed of said last stand.
 3. The method ofclaim 1 wherein said gauge deviation signal is multiplied by thecross-sectional area of the strip material between said last two standsto derive said first error signal and said first error signal ismultiplied by the speed of said last stand to derive the second errorsignal.
 4. The method of claim 1 wherein said gauge percent deviationsignal is summed with said first error signal to produce a third errorsignal, and including the steps multiplying said third error signal bythe speed of said last stand to derive said second error signal, andrunning said second and third error signals and using the sum to controlthe speed of said last stand.
 5. The method of claim 1 including thestep of producing a signal proportional to the cross-sectional area ofthe strip material between said last two stands by multiplying the gaugeof the strip material between the first two stands in said tandemrolling mill by the speed of strip material as the output of the firststand in the rolling mill, and dividing the product by the speed of thefourth stand in the tandem rolling mill.
 6. Apparatus for controllingthe final output gauge of strip material passing through a tandemrolling mill by varying the tension of the strip material between thelast two stands in said mill, comprising an automatic gauge control loopincluding means for measuring the gauge of strip material issuing fromsaid last stand at a point removed from the last stand and for producinga signal proportional to the actual measured gauge, means for comparingsaid actual gauge signal with a desired gauge signal to derive a gaugepercent deviation signal based on desired delivery gauge, means forvarying the speed of said last stand to thereby vary the tension betweenthe last two stands of the tandem rolling mill, means for generating across-sectional area signal which varies as the cross-sectional area ofthe strip material between the last two stands of the mill varies, meansfor generating a signal which varies as a function of the speeD of thelast stand in the mill, means for modifying said gauge percent deviationsignal as a function of variations in said cross-sectional area signaland said speed signal, and means for applying said modified gaugepercent deviation signal to said means for varying speed whereby thespeed of the last stand will be modified in response to changes in bothstrip cross-sectional area and strip speed.
 7. The apparatus of claim 6wherein said means for modifying comprises means for multiplying saidgauge percent deviation signal by the cross-sectional area of the stripmaterial between said last two stands to derive a first error signal,means for multiplying said first error signal by the speed of said laststand to derive a second error signal, and means for combining saidfirst and second error signals and for applying the combined errorsignals to said means for varying speed.
 8. The apparatus of claim 6including a speed regulator for said last stand, tension regulatingcircuit means having its output connected to said speed regulator, andmeans for applying the modified gauge percent deviation signal to saidtension regulating circuit means.